What’s wrong with flat tax?

For the life of me, I cannot understand why all the semi-literate politicians in D.C. spend their life trying to look good and institute a low tax for everyone. What is wrong with a progressive flat tax for everyone with no deductions or exclusions for anything? Those earning less than $50,000 and $150,000 pay a little more than those above that pay a little more. We could do away with a significant number of the IRS folks. This would not be appropriate for most corporations, etc., but for the majority of individual Americans, it would be fair, easy and less cumbersome for most who do not try to cheat the government.

Can anyone tell me what the real downside is to this that would not be offset by the chaos of filling a tax return under our present system? I’d be glad to learn.